Where continuously operating sugar centrifugals are concerned, it often matters, subject to their use, to separate from the massecuite even crystals having a relatively small grain size as, for example, in the processing of low-grade massecuites, because fine crystals not retained by the working screen would pass, together with the run-off into the final molasses, thus reducing the sugar yield. However, even in the processing of middle-grade massecuites, the object is to have the smallest possible quantity of sugar crystals pass into the run-of syrup.
Experiences have shown that with regard to the retention of the fine crystals the working screens of continuously operating sugar centrifugals should have slotted holes of about 0.04 to 0.06 mm width. The sides of the slotted holes should be as smooth as possible and these sides must widen the slot in the direction of the liquid passage in order to avoid clogging and rear incrustation.
Another criterion for the utility of the working screens is their relative open screening area which affects the throughput of the respective continuously operating centrifugal. The amount of liquid which can be separated by a screen per unit of time, with all other circumstances being the same, is the larger, the larger the sum of the screen hole cross-sections is per unit of screening area. Hence, the open screening area has a throughput-limiting influence on the centrifugal. With the processes hitherto used for the manufacture of working screens for continuously operating centrifuges it is possible, however, to produce screens only having an open screening area of about 6.5% at best, if at the same time a screen thickness shall be ensured which is satisfactory at least to some extent as far as the useful operating life of such screens is concerned.
By punching, a process which is used only in exceptional cases for the manufacture of working screens for continuously operating sugar centrifugals, high-grade steel screens having a maximum thickness of 0.18 mm and an open screening area of only 5.5% may be manufactured. Moreover, special auxiliary steps must be taken in this process to produce conical screen holes. The attainable open screening area of these screens is as unsatisfactory as the relatively small screen thickness. Besides, punching results in rough cutting surfaces and burrs, which increases the risk of clogging as a result of incrustation. The slotted area of these screens must be subjected to a bending and upsetting action in order that the screens may be manufactured at all by punching. Therefore, the exactness of the slot contours and dimensions is adversely affected. Both, the slot contour and the slot dimension, however, are essential factors of the separating characteristics.
As compared to the punching process, the electroforming process chiefly used hitherto for the manufacture of working screens has the advantage of ensuring very smooth surfaces and exact slot contours. Furthermore, it is possible to manufacture screens of 0.24 to 0.28 mm thickness with a slot width of about 0.06 mm. Compared to punched screens, this represents an increase in screen thickness of about 0.1 mm resulting in a corresponding increase in service life. Moreover, these electroformed screens are superiors to punched screens because with their open screening area of 6.0 to 6.5% the open screening area is larger by about 1%. These values, however, are the limits of the electroforming process as it is known hitherto. Though larger open screening areas are attainable, this is possible only when the screen thickness is reduced at the same time due to the given growth laws of electrodeposition on matrices. From an electro-conductive spot on the surface of a matrix, material is deposited in a substantially uniform manner both horizontally and vertically. However, a screen slot is bounded by two edges. Thus, the material spreads from both edges into the open screen slot during depositing. When, for example, a screen having a thickness of 0.1 mm and screen slots of 0.1 mm width shall be manufactured, the screen slot edges lying on the matrix must be spaced apart 0.1+2.times.0.1=0.3 mm. If this spacing is not maintained, the resulting screen slots will be too narrow. If, under the same prerequisites material is deposited up to a thickness of 0.15 mm, the screen slots have grown closed. Thus, the spacing between the screen slot edges on the matrix has to be the greater, the thicker the screen is to become. Therefore, the spacing between neighboring screen slots becomes the greater, the thicker the screen is and the open screening area decreases as the screen thickness increases.
In the past, electroformed working screens for continuously operating sugar centrifugals have been put on the market. However, due to these facts, although the open screening area exceeded 6.5%, at least some of these screens had a thickness substantially less than 0.2 mm. These screens had to be reinforced at the rear by supporting screens, against the stress caused by the squeezing load caused by the massecuites sliding over the working area under the high gravity field of the centrifugal.
These supporting screens do not only have the disadvantage to cause an additional expense, they also reduce the effective screening area because they cover part of the rear of the screen slots of the working screen. The increase in open screening area obtained at the price of a reduction of the screen thickness is reduced again by the need for a supporting screen due to the small screen thickness. Furthermore, the additional supporting screen arranged behind the working screen results in an aggravation of the flow conditions and increases the risk of incrustation.
In spite of the supporting screen, these "thin" working screens are more delicate and vulnerable in use so that they have to be replaced more frequently than screens of normal thickness. Any screen replacement always involves a down time of several hours. Therefore, the known "thin" screens having a larger open screening area do not represent a satisfactory solution.